Ultra-wideband technology has several unique and attractive features, such as immunity to multi-path interference, jamming, and intentional and unintentional interference, low probability of intercept, and enhanced penetration capabilities. Additionally, the elimination of radio frequency (RF) components from the UWB transceivers enables the use of devices with relatively low hardware complexity. Nanosecond pulse durations also make it attractive for application such as precision geo location. As a result, UWB technology is currently being considered for the support of robust, scalable networks that will support a wide range of military and commercial applications in harsh environments (e.g., high multi-path fading, high jamming, and high interference environments).
UWB technology also referred to as impulse, baseband, and zero-carrier technology, uses ultra short pulses, typically less than a nanosecond in duration, to convey information. In the communications context, this property implies that the transmitted signals are highly distorted after bouncing off surrounding objects. Compared with a nano-second pulse duration, the multi-path delay spread is usually of the order of hundreds of nanoseconds.
Since UWB transmitters emit signals at levels below the noise floor, UWB signals have a low probability of detection and a low probability of intercept. Spread spectrum techniques are well suited to extract UWB signals under these circumstances. In spread spectrum techniques, the frequency components of the signal are “spread” across the frequency spectrum by encoding each bit of information in a symbol consisting of a series of “chips” that are transmitted during a symbol period that is allotted for each bit of information.
While the UWB properties described above are desirable for covert communications and help reduce interference, they make it difficult to demodulate and decode the signal. Three binary modulation schemes are often considered for UWB communication. The first scheme is binary antipodal signaling where the polarization of the transmitted pulse is determined by the modulated bit/chip. Thus, the pulse changes state from −1 to +1 depending on the binary value of the modulated bit/chip. Unfortunately, binary antipodal signaling provides adequate performance only when the distortion to the signal is minimal. In addition, adequately resolving the polarization of the received signals is a particularly challenging issue in a high multi-path delay spread environment. Typically, complex receivers are required to resolve the polarization of each pulse, thereby adding to the cost and complexity of the system.
Another modulation scheme considered for UWB communication is on off keying (OOK), where a modulated bit/chip is resolved by determining whether or not the pulse is present. Although OOK modulation is simple to implement for both modulation and demodulation, OOK is vulnerable to additive noise in the channel.
A third modulation scheme that has been considered is pulse position modulation (PPM), which provides more immunity to signal distortion and additive noise. Conventional PPM typically assumes a fixed timing offset between pulses in the signal set. This has several drawbacks for UWB systems. Since the maximum delay of dense multi-path channels are on the order of hundreds of nanoseconds, the spacing of conventional PPM pulses are required to be of the same order of magnitude to minimize inter-pulse interference and maximize the probability of decoding. In TH-CDMA, a large spreading gain and a large number of time bins to hop the modulated signal are employed in conjunction with PPM. As a result, the large PPM modulation pulse spacing can severely limit system throughput.
Therefore, there is a need in the art for a modulation scheme that realizes the benefits of standard pulse position modulation while providing greater immunity to multi-path spreading. In particular, there is a need for a novel pulse position modulation scheme that is more robust to multi-path effects while allowing for a greater throughput in UWB systems than conventional pulse position modulation schemes.